1. Field of the Invention
The invention relates to a communication system for communicating data by using an interface comprising an upper layer and a lower layer, an information processing apparatus, an output apparatus, a control method, and a memory medium.
2. Related Background Art
Hitherto, a PC parallel interface added with a plurality of modes for providing a high speed bidirectional communication between devices has been defined in IEEE 1284. In IEEE 1284, however, a protocol called a “transport protocol” for a Point-to-Point link is not defined. In IEEE 1284.4, a Point-to-Point protocol regarding several layers locating at intermediate stages between a physical interface and an application layer is designated. Such a protocol has functions and characteristics of a transport layer and a session layer in the OSI model. Further, a method of performing a bidirectional communication as a data communication between a host and a printer on the basis of IEEE 1284.4 has been proposed.
IEEE 1284.4 is a packet protocol to simultaneously transmit and receive a plurality of information (data and control information) by using one Point-to-Point link connected between two devices. The protocol, however, provides a flow control and a multiplexing service at a basic transport level instead of control information (control command) of the device. Multiplexed logical channels are independent and even if a certain channel is closed, the other channels are not influenced. Although the protocol presumes that it is operated on the IEEE 1284 interface, it can be operated on another Point-to-Point interface.
When an IEEE 1284.4 error occurs, however, the side which detects the error occurrence has to transmit an Error command specified in IEEE 1284.4 and an Exit command specified in IEEE 1284.4 to the communication partner side, receive an ExitReply command specified in IEEE 1284.4, and finish the IEEE 1284.4 mode. To resume the communication by IEEE 1284.4, the side which detected the error has to finish the communication once by IEEE 1284.4 and start the communication again by an Init command specified in IEEE 1284.4.
It is not guaranteed that the transmission and reception of the commands in an error occurring situation succeed and there is also a case where the side which detected the error has to wait for the ExitReply command permanently.
In an output apparatus having a function which does not use a bidirectional function, it the cause of the IEEE 1284.4 error is that the bidirectional function is not used during the operation of IEEE 1284.4, the ExitReply command is never issued from the output apparatus.
Among output apparatuses having an automatic switching apparatus of the IEEE 1284.4 mode and the IEEE 1284 mode (generally, referred to as a Centronics), as for the output apparatus which operates in the IEEE 1284 mode at the time of turn-on of a power supply, if the cause of the IEEE 1284.4 error is a turn-off and a re-turn-on of the power supply, the Error command and Exit command are erroneously recognized as a part of print data in the IEEE 1284.4 mode.